# Bubble chart

A bubble chart encodes a third quantitative measure as the area of each point. It is genuinely useful and easily overdone: area is harder to read than position.

Use size for the measure you care about least of the three.

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## When this is the right chart

* Three measures where two matter more than the third.
* Portfolio views: effort, impact and cost.
* Comparing items where scale differs enormously.

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## What the data needs to look like

| **Field** | **Type**          |
| --------- | ----------------- |
| X         | Quantitative      |
| Y         | Quantitative      |
| Size      | Quantitative      |
| Colour    | Nominal, optional |

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## A worked example

Effort, impact and team size in one chart:

![Bubble chart](https://help.gocapable.com/images/att1418494053.svg)

The source, which you can paste into a new diagram and edit:

```
{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Roadmap items: effort, impact and team size",
  "width": 380,
  "height": 280,
  "data": {"values": [
    {"item": "Single sign-on", "effort": 8, "impact": 9, "people": 4, "area": "Platform"},
    {"item": "Bulk export", "effort": 3, "impact": 7, "people": 2, "area": "Apps"},
    {"item": "Dark mode", "effort": 4, "impact": 5, "people": 2, "area": "Apps"},
    {"item": "Audit log", "effort": 6, "impact": 8, "people": 3, "area": "Platform"},
    {"item": "Legacy import", "effort": 9, "impact": 3, "people": 5, "area": "Data"},
    {"item": "Custom themes", "effort": 7, "impact": 2, "people": 3, "area": "Apps"}
  ]},
  "mark": {"type": "circle", "opacity": 0.75},
  "encoding": {
    "x": {"field": "effort", "type": "quantitative", "title": "Effort", "scale": {"domain": [0, 10]}},
    "y": {"field": "impact", "type": "quantitative", "title": "Impact", "scale": {"domain": [0, 10]}},
    "size": {"field": "people", "type": "quantitative", "title": "People"},
    "color": {"field": "area", "type": "nominal", "title": "Area"},
    "tooltip": [{"field": "item"}]
  }
}
```

---

## A few things that catch people out

* Size is read as area, not radius. Encoding it as radius exaggerates large values fourfold.
* Overlapping bubbles need opacity, or the small ones vanish.
* Label the bubbles that matter. A legend of eight items is a lookup task.

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## Related

[HistogramThe shape of a distribution.](https://help.gocapable.com/diagrams/histogram.html)

[Heat mapIntensity across two categorical axes.](https://help.gocapable.com/diagrams/heat-map.html)

[Charts and graphsBack to the chart overview.](https://help.gocapable.com/diagrams/charts-and-graphs.html)

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_Pick the chart for the question, not for the look._